1. Field of the Disclosure
The disclosure relates generally to resonator devices characterized by a feedthrough capacitance and, more specifically, to devices having a compensation capacitance to offset the feedthrough capacitance.
2. Brief Description of Related Technology
Resonators are used in a variety of devices and applications involving the generation of a signal with an accurate and stable frequency. For example, oscillators have utilized resonators to generate clock signals of a reliably stable frequency. The frequency stability of an oscillator is often assessed in two ways—phase noise characteristics for short-term stability and aging characteristics for long-term stability. With their high frequency stability in both the short term and the long term, quartz crystal resonators have often been relied upon to meet the more stringent phase noise specifications of frequency reference and master clock applications.
Micromechanical or MEMS (microelectromechanical systems) resonators have been developed as an alternative to quartz crystal resonators for oscillator and other devices and applications. MEMS resonators are typically electrostatically actuated, but can also be actuated piezoelectrically, magnetically, and optically. For electrostatically actuated resonators, a resonant element of the MEMS device is spaced from a stationary drive electrode such that the MEMS device is coupled across an air gap. The coupling is a function of the time-varying capacitance resulting from the device vibration.
Unfortunately, the spacing between the elements of a MEMS resonator presents a feedthrough, static capacitance in addition to the time-varying capacitance. This feedthrough capacitance arises from the resonant element not vibrating the entire extent of the air gap, and generally allows a portion of the drive signal to pass through the device. As a result, the efficiency of the MEMS resonator is adversely impacted. For oscillator devices, overall stability may be compromised, as the oscillator may be allowed to operate at a parasitic resonance not under the influence of the primary device resonance. More generally, the static capacitance undesirably provides a feedthrough path from the input to the output, degrading performance with decreased frequency accuracy, increased phase noise and an increased noise floor.
Previous attempts at improving performance have been directed to altering the physical structure of the MEMS resonator to minimize the feedthrough capacitance. Unfortunately, altering the structure of the vibrating and other components of the MEMS resonator may introduce other complications and complexities.